Ad homs are a rhetorical tactic used by the logically challenged and the factually deficient who are in denial about losing an argument. Always have been; always will be.
If the shoe fits, the Mann should wear it.

If you can’t win on the facts, attack the opponent. The AGW crowd has co-opted the language. Global warming morphed into “climate change” making any deviation from some norm something to be feared. I assume if you are fighting climate change, then you believe the climate should be static and also believe you can control it. I plan to do all I can to address climate change. I plan to very slowly adapt.

There is a place for ad hominem attacks. For example, we of the JS crew mostly believe that Michael Mann’s work is at best mistaken, at worst a knowing lie. We say so, calling Mann himself a charlatan because his work shows him to be one. It’s still an ad hominem attack. Same for Al Gore, James Hansen, and their enablers.
And of course we are the objects of ad hominem attacks, or our public figures are at least.
In itself, calling someone a charlatan proves nothing. If you can back up the accusation with facts — as we can with Hansen and Gore and Mann — then the ad hominem gains some validity.
In the specific column that Mann attacks and Mr. Milloy links, the items stated as facts are facts and not disputable. The areas one might dispute are areas of opinion. Happens I agree with the writer but it’s still opinion and not claimed as anything else.
Compare and contrast to the legions of reporters with a shortcut to enter “carbon dioxide, the primary global warming gas” or something similar, which is a statement false to fact. The primary global warming “gas” is water vapor. The primary global warming gas is nitrogen, 78% of the atmosphere.